This invention relates to a plesiochronous buffer circuit which is for use in an earth station in coupling a domestic digital network and a satellite network, which is typically a TDMA (time division multiple access) satellite network.
A plesiochronous buffer circuit comprises a buffer memory, which is often called a doppler buffer memory in the art. The buffer memory is for compensating during transmission and reception of a PCM (pulse code modulation) signal for a clock frequency difference between clock frequencies used in the domestic digital and the satellite networks.
Clocks of the clock frequency used in the domestic digital network, are herein called station clocks. Those of the clock frequency used in the satellite network, are called satellite clocks. A typical PCM signal comprises a plurality of frames each of which comprises a particular data signal.
A conventional plesiochronous buffer circuit is disclosed for use in buffering the particular data signal by Yasuo Hirata et al in a Japanese technical paper, "Kokusai Tusin no Kenkyu" (Study of International Communication) No. 114 (October 1982), pages 25-39, under the title of "Pureziokuronasu Mokan Setsuzoku-sochi no Kaihatsu" (Development of a Coupling Device between Plesiochronous Networks). The buffer circuit of Hirata et al comprises a write controlling arrangement which controls writing of the buffer memory for the particular data signal in accordance with the satellite clocks when the PCM signal is received through the satellite network. A read controlling arrangement controls reading of the buffer memory for the particular data signal in accordance with the station clocks. A comparing circuit compares writing and reading phase signals which are produced by the write and the read controlling arrangements, respectively. The writing and reading phase signals are produced at write and read timings of the particular data signal in and from the buffer memory, respectively. The comparing circuit produces a slip control signal when the writing and the reading phase signals become nearer to each other than a preselected time interval. The read controlling arrangement responds to the slip control signal and carries out a slip control such that the particular data signal is read from the buffer memory at a slipped timing displaced relative to the read timing by one frame. That is, either repeat or deletion of one frame is carried out for the particular data signal by the slip control.
It is a recent trend that a PCM signal of a different format is used in a communication network. The PCM signal comprises a plurality of multiframes each of which comprises a predetermined number of frames. Each frame comprises first and second parts to which information is assigned at every multiframe and at every frame, respectively. More particularly, the first part of a predetermined one of the frames of each multiframe comprises a multiframe synchronization or alignment signal. The first part of each of the other frames of each multiframe comprises a first data signal. The second part comprises a second data signal which corresponds to the above-mentioned particular data signal. In each frame, the first and the second data signals appear in different periods. A significant word is defined in each multiframe by the first parts of the predetermined number.
On reception of the PCM signal of the different format, the buffer circuit of Hirata et al is capable of carrying out the slip control for the second part to reduce distortion of the second data signal. This is because either repeat or deletion of one frame is carried out for the second part to which information is assigned at every frame. However, the repeat or the deletion of one frame is not applied to the first part to which information is assigned at each multiframe. This is because the number of the frames of each multiframe is defined as the predetermined number. When the repeat or the deletion of one frame is applied to the first part, the number of the frames of each multiframe becomes different from the predetermined number.
According to INTELSAT EARTH STATION STANDARDS (IESS), Document IESS-309 published on July 1, 1985, any slip shall be a multiple of one frame period. In order to realize the slip control for the first and the second parts, attempts may be made to repeat and delete one multiframe in view of the standard set up in the Document IESS-309. Such repeat and deletion of one multiframe may enable a reduction of distortion of the first data signal assigned to the first part. However, distortion is inevitable for the second data signal assigned to the second part at every frame.